Treatment of effluents

ABSTRACT

The present invention relates to removal of organic pollutants from aqueous effluent streams. The present invention provides a method of destructively oxidising an organic compound present in an aqueous solution, the method comprising oxidising the organic compound in the presence of a catalyst which contains uranium. The catalyst may comprise a uranium oxide. The reaction may be carried out at low temperature, e.g. ambient temperature. The method may be used to treat aqueous effluent streams to remove organic compounds from the stream.

The present invention relates to removal of organic pollutants fromaqueous effluent streams.

In many industries, eg the chemical and nuclear industries, considerablevolumes of aqueous effluent are produced which require treatment tominimise the concentrations of pollutants before discharge to theenvironment. One class of pollutants in particular which require removalare organic compounds. One way of removing organic compounds is byoxidising the compounds, eg to CO₂.

Destructively oxidising organic compounds in aqueous streams can beachieved in a number of ways. Steam stripping may be used initially toremove volatile compounds from aqueous streams. The compounds may thenbe destroyed in the gas phase.

Although volatile compounds can be treated in this way, non-volatilecompounds remain in the liquid effluent and must be treated by adifferent technology.

This may be achieved by processes such as non-catalytic or catalytic wetair oxidation in which the effluent is heated, eg above 200° C., underelevated pressure so that the system remains liquid. The need tomaintain the effluent at high temperature and pressure is obviouslyundesirable. Also the typical limits on the concentration of organiccompounds in solution which can be treated by the method are in therange 2-20%. Therefore, other methods must be found to treat effluentswith lower concentration levels than this.

Lower temperature oxidation of organic compounds in aqueous solutionscan be achieved through the use of catalysts. To date, numeroushomogenous catalyst systems have been used. These have included, forexample, mixtures of hydrogen peroxide and FeCl₂ (Fenton's reagent) ormixtures of hydrogen peroxide and chromium(VI) solutions.

However, problems can arise in such homogeneous catalyst systems. Forexample, since the catalyst is in the same phase as the solution, aproportion of the dissolved catalyst itself may be discharged to theenvironment along with the treated effluent leading to loss of valuablecatalyst and accumulation of catalyst in the environment. Furthermore,although many homogeneous catalytic systems are generally efficient atorganic removal at moderate concentrations, at low concentrations ofpollutants, such as several ppm, their efficiency can be significantlyreduced.

Heterogeneous catalysts, such as metals or metal oxides, are morecommonly used to catalyse reactions of gas phase organic compounds andhave been used less widely in the aqueous phase. Problems are likely tobe encountered when using such catalysts to catalyse reactions in theaqueous phase. For example, such catalysts may dissolve in the aqueoussolution. Also, contact between the organic compound and the catalystmay be less effective at low concentrations of the compound due tointerfering effects of the water molecules. In the case of supportedcatalysts, the support material, such as silica or alumina, may becomehydrolysed which further reduces the ability of the organic compound tocontact the catalyst.

Nevertheless, titanium dioxide, TiO₂, has been used in thephotocatalytic oxidation of organic compounds in which ultraviolet lightis used to excite electrons across the band gap in TiO₂ and promoteoxidation of the organic compound. However, the efficiency of thesecatalysts is low.

In view of the problems detailed above, it can be seen that the needexists for improved methods of organic destruction.

According to the present invention there is provided a method ofdestructively oxidising an organic compound present in an aqueoussolution, the method comprising oxidising the organic compound in thepresence of a catalyst which contains uranium.

Desirably, the catalyst comprises a uranium oxide with a stoichiometryfrom UO₂ to UO₃ inclusive. The catalyst may, for example, comprise UO₂,U₃O₈, UO₃ or another uranium oxide. The catalyst may comprise a mixtureof two or more such uranium oxides. Preferably the catalyst comprisesU₃O₈.

The catalyst may additionally contain one or more other metals. The oneor more other metals may comprise, for example, vanadium, iron, copperor platinum.

The one or more other metals may be present as metal oxides, forexample, vanadium, iron, copper or platinum oxides.

The catalyst may, for example. comprise a uranium oxide as defined aboveand one or more other metals. The one or more other metals may forexample be vanadium, iron, copper or platinum or oxides thereof. Acatalyst containing uranium and vanadium has been found to beparticularly effective for destroying organic compounds in the presentinvention.

The catalyst may comprise a mixed metal oxide. The catalyst may comprisea mixture of single phase metal oxides.

The catalyst may comprise a mixed metal oxide which includes uranium andat least one other metal. The uranium-containing mixed metal oxide maybe present with a uranium oxide and/or one or more other metals or metaloxides.

The catalyst may be supported on a support, for example, silica (SiO₂),alumina (Al₂O₃), zeolites, activated carbon, titania (TiO₂), zirconia(ZrO₂) or ceria (CeO₂).

The catalyst need not comprise a uranium oxide, but may comprise anotheruranium containing substance.

It has been found that oxides of uranium have a high catalytic activityfor the oxidative decomposition of organic compounds in aqueoussolutions. It has also been found that the activity may be enhanced insome cases by the incorporation of other metals or metal oxides in thecatalyst. Different metals or metal oxides may have different effects onthe activity of the catalyst for destroying a particular organiccompound. One or more particular metals or metal oxides may beincorporated in the catalyst to enhance the destruction of anyparticular organic compound. Thus the composition and form of thecatalyst may be tailored towards the particular organic pollutant whichit is desired to destroy.

Advantageously, in the method according to the present invention, thecatalyst does not dissolve in the aqueous solution. Since the integrityof the catalyst is maintained throughout the reaction, the catalyticactivity is likewise maintained and fresh catalyst is not required to beadded frequently. Also, the problems in homogeneous catalytic systemsrelated to dissolved catalyst being washed away with the effluent areminimised.

The method according to the present invention is also highly efficient.It has been found that in excess of 99.9% of an organic compound in anaqueous effluent may be destroyed.

The organic compound may be present in a concentration above or belowits solubility limit in water.

In contrast to some prior art methods of destroying organic compounds,the oxidation reaction in the present invention may be carried out atrelatively low temperatures. The reaction may be carried out below 100°C. Preferably the reaction is carried out below 50° C. to avoid theproblems associated with heating large volumes of water. The inventorshave found that the reaction is efficient even when carried out atambient temperature. Since the cost of heating large volumes of effluentcan be very high in practice, the high efficiency of the methodaccording to the present invention at ambient temperatures provides asignificant advantage for industrial scale applications.

The oxidant employed in the reaction may be a commonly used oxidant suchas hydrogen peroxide. The inventors have also found that air or oxygenis an efficient oxidant in the present invention. Both hydrogen peroxideand air have the advantage that they decompose to environmentally benignproducts. It is envisaged that many other oxidants may be used equally.For example, KMnO₄ or ozone may be used. Other oxidants such as H₂O₂ andsodium hypochlorite mixtures, sodium peroxydisulphate and calciumhypochlorite may be efficient oxidants but may present their ownenvironmental hazards and for that reason may be less favoured.

It will be appreciated immediately that the ease and low cost of beingable to use air as an oxidant and operate at ambient temperaturerepresents a considerable advantage.

The invention may be carried out as either a batch or continuous flowprocess.

The types of organic compounds which may be destroyed in the methodaccording to the present invention include alkanes, alkenes, alkynes,aromatics, alcohols, aldehydes, ketones, carboxylic acids, esters,ethers, amines, detergents, organophosphates and derivatives of allthese including compounds containing substituent groups and heteroatoms.In particular, the inventors have found that alkyl phosphates such astributyl phosphate (TBP), sodium di-butylphosphate (NaDBP) and hydrogenmonobutyl phosphate (HMPB) are efficiently destroyed. It should beunderstood that the foregoing list is not exhaustive and does not limitthe invention in any way. In principle, all types of organic compoundmay be destroyed by the method according to the present invention.Several different organic compounds present in the same aqueous effluentmay be treated together.

Particular industries each have their own array of problem compounds todeal with. The present invention is potentially applicable to all typesof organic compound. In the nuclear industry for example, outflows fromuranium processing facilities may contain a number of contaminantorganic compounds such as tributyl phosphate (TBP), odourless kerosene(OK), ethylenediaminetetraacetic acid (EDTA) and citric acid. Thepresent invention may be effective in destroying all of these compoundsin aqueous solution, particularly at trace concentrations.

Embodiments of the present invention will now be described by way of thefollowing Examples. The Examples are merely illustrative of theinvention and do not limit the invention in any way.

The Catalysts Used in the Examples

a) Single Oxide Catalysts

The single oxide catalysts used in these studies were U₃O₈ and Co₃O₄.The U₃O₈ was prepared by ramping the temperature of a starting materialof uranyl nitrate in an air atmosphere to 300° C., holding for 1 hour at300° C., ramping to 800° C. and then holding for 3 hours at 800° C. TheCo₃O₄ was used as supplied and did not undergo any form of chemical orthermal pre-treatment.

b) Silica Supported Catalysts

A series of uranium containing catalysts, based on a system of silicasupported uranium oxide has been prepared. These catalysts were preparedby an incipient wetness technique described by G C Bond, HeterogeneousCatalysis Principles and Application, Clarendon Press, Oxford, 2ndEdition 1987, p101. This procedure entailed the addition of a minimumamount of a solution containing the supported ions to the supportmaterial. Initial catalysts were 10 mol % U on SiO₂ prepared by thefollowing method. SiO₂ was impregnated with a minimum quantity ofsolution of UO₂(NO₃)₂.6H₂O dissolved in distilled water. Prior to use,the catalysts were calcined by a two stage process which involvedheating the precursors in static air for 1 hour at 300° C. and then fora further 3 hours at 800° C. This calcination process decomposedresidual nitrate on the support and formed the supported oxides.

c) Alumina Supported Catalysts

Alumina supported uranium oxide catalysts were prepared by dissolving2.46 g (10 mole % w.r.t. alumina) of UO₂(NO₃)₂.6H₂O (uranyl nitrate) and2.94 g urea (100 mole % w.r.t. alumina) in water and adding this slowlyto 5 g of alumina (Aldrich, activated, neutral, Brockman 1, std. Grade,CA 150 mesh) with strong mixing. This mix was then heated to 70-80° C.for 3 hours with occasional stirring. It was then heated at 110° C.overnight. It was calcined by heating to 300° C. (ramp rate=10° C./min),held at this temperature for 1 hour and then heated to 800° C. (sameramp rate) and held for 3 hours.

d) Uranium/Vanadium Oxide Catalysts

(i) Alumina Supported Uranium/Vanadium Oxide Catalysts

The metal salts uranyl nitrate (0.4922 g) (5 mol % wrt alumina) andammonium vanadate (0.1147 g) (5 mol % wrt alumina) were dissolved in 2ml water (0.6 ml of conc. nitric acid was added merely to help dissolvethe ammonium vanadate and this was not added when other metal salts wereused). The solution was then added to 2 g alumina (activated, neutral,Brockmann 1, Std Grade, ca. 150 mesh) and the mixture was stirred whileheated to remove the water.

When the mixture was dry, it was heated to 300° C. at a ramp rate of 10°C. per minute and held at 300° C. for 1 hour. The temperature was thenincreased to 700° C. (at the same ramp rate) and held at thistemperature for 3 hours. All heating was carried out in ambient air. Thesample was then removed, cooled to room temperature and crushed to apowder.

(ii) Silica Supported Uranium/Vanadium Oxide Catalysts

Silica supported uranium/vanadium oxide catalysts were prepared in thesame way as the corresponding alumina supported catalysts describedabove subject to the following differences: 2 g of silica was used inplace of the 2 g alumina and the metal salts were dissolved in 6 mlswater and 1 ml nitric acid prior to the addition of the silica.

(iii) Unsupported Uranium/Vanadium Oxide Catalyst

The metal salts uranyl nitrate (3 g) and ammonium metavanadate (0.0777g, 11 mol % wrt uranium) and 2 g ammonium nitrate were dissolved in theminimum amount of water. Because ammonium metavanadate was used, a smallamount of nitric acid (0.2 ml) was added to aid dissolution. The waterwas evaporated with stirring. The resulting mixture was powdered.

The calcination procedure was as described for the supporteduranium/vanadium materials.

e) Uranium/Iron/Copper Catalysts

The metal salts uranyl nitrate (0.2788 g, 3.3 mol % wrt silica), ironnitrate (0.2244 g, 3.3 mol % wrt silica), and copper nitrate (0.1294 g,3.3 mol % wrt silica), were dissolved in 3.8 mls of distilled water andthe solution was added with stirring to 1 g silica to form a gel. Thegel was dried at 110° C. overnight and then crushed to form a powder.The powder was heated in air from room temperature to 300° C. at a ramprate of 10° C./min. It was held at 300° C. for an hour and then thetemperature was increased to 650° C. at the same ramp rate. It was heldat 650° C. for 3 hours and then removed from the furnace, cooled andpowdered.

Catalysts comprising U/Fe and U/Cu were also prepared as describedabove, except that 5 mol % of the metals were used wrt silica.

f) Catalyst Characterisation

Catalysts were characterised by X-ray diffraction.

In the cases of both U₃O₈ and U₃O₈ on alumina there were no significantchanges to the XRD patterns before and after use. Matching the patternswith the JCPDS powder diffraction file showed α-U₃O₈ to be the onlysignificant phase present.

Major changes were evident for the silica supported uranium catalyst inthe before and after patterns. The unused catalyst was found to consistof α-U₃O₈, whereas the used sample gave a good match for UO₂.hydrate.

The precise structure of the uranium/vanadium oxide catalysts wasdifficult to determine from the XRD results but the results indicatethat the materials comprise mostly a mixed uranium/vanadium oxide withsome discrete phases of uranium oxide and vanadium oxide.

Similarly, the structure of the uranium/iron/copper oxide catalystscould not be determined precisely but the results indicated the presenceof both mixed oxide phases and discrete single oxide phases.

General Procedure for Examples 1-3

Reactions were carried out in a stirred batch reactor using aqueoushydrogen peroxide (30% w/w in water) as the oxidant unless otherwisestated. Various organic compounds were used to model problematicpollutants. A substrate mix A was prepared comprising 2.1 g of benzene,2 g of chlorobenzene, 2 g of mixed hexanes, 250 g of water and 20 g oft-butanol. The t-butanol was added merely to aid miscibility and is notessential to the invention. Amounts of catalyst (0.1-0.2 g) and amountsof H₂O₂ (where used as oxidant) were added to 5 or 10 ml batches of mixA. The H₂O₂ used was a 30% solution in water. The reaction was thencarried out for 16 hours at either ambient temperature or 50° C. asspecified below. The catalysts tested were unsupported and supportedU₃O₈ and unsupported Co₃O₄. In addition a control experiment using nocatalyst was carried out. A Varian 3400 GC fitted with SE 56 column anda flame ionisation detector was used to detect the product. The amountof conversion of the organic compound was calculated from the peak areasof the gas chromatograph.

EXAMPLE 1

Mixture of Benzene, Chlorobenzene and Hexanes at Ambient Temperature

Batches (5 or 10 ml) of mix A were treated according to the generalprocedure detailed above at room temperature using H₂O₂ as oxidant.Separate treatments were carried out using unsupported U₃O₈,SiO₂-supported U₃O₈ and Al₂O₃-supported U₃O₈ as the catalysts. Inaddition, a control treatment was carried out without a catalyst. Theresults are summarised in Table 1 below.

The uncatalysed reaction showed no conversion of organic compound. Theunsupported U₃O₈ catalyst, however, was highly active and near 100%conversion was observed. The supported U₃O₈ catalysts also showed highactivity and were only slightly less active than the unsupported U₃O₈.

TABLE 1 Results for the oxidative destruction of hexanes, benzene andchlorobenzene in water using hydrogen peroxide at ambient temperatureand the catalysts stated. Substrate Catalyst Oxidant % Conversion Mix ANo Catalyst H₂O₂ Hexanes: 0 5 mls 2 mls Benzene: 0 Chlorobenzene: 0 MixA U₃O₈ H₂O₂ Hexanes: >99 5 mls 0.2 g 2 mls Benzene: >99Chlorobenzene: >99 Mix A U on SiO₂ H₂O₂ Hexanes: >99 10 mls 0.1 g 1.5mls Benzene: >90 Chlorobenzene: >90 Mix A U on Al₂O₃ H₂O₂ Hexanes: >9510 mls 0.1 g 1.5 mls Benzene: >99 Chlorobenzene: >99 Mix A Co₃O₄ H₂O₂Hexanes: <10 5 mls 0.2 g 2 mls Benzene: <10 Chlorobenzene: <10

The effectiveness of the catalysts in the method according to thepresent invention is especially remarkable when compared to otherheterogeneous catalysts, such as Co₃O₄, which are known to oxidiseorganic compounds in the gas phase. To illustrate this, experiments werecarried out with Co₃O₄ under identical conditions to the U₃O₈experiments above but little conversion of the organic compounds wasfound, as shown in Table 1.

EXAMPLE 2

Mixture of Benzene, Chlorobenzene and Hexanes at 50° C.

Batches of mix A were treated according to the general proceduredetailed above at 50° C. The reactions were carried out in a stainlesssteel (s.s) bomb. The catalyst studied was unsupported U₃O₈. A separatecontrol treatment was carried out without a catalyst. The results areshown in Table 2 below.

TABLE 2 Results for the oxidative destruction of hexane, benzene,chlorobenzene in water using hydrogen peroxide at 50° C. and U₃O₈ as thecatalyst. Substrate Catalyst Oxidant % Conversion Mix A No H₂O₂ Hexanes:85 5 mls Catalyst 2 mls Benzene: 84 Chlorobenzene: 92 Mix A U₃O₈ H₂O₂Hexanes: >90 5 mls 0.2 g 2 mls Benzene: >99 Chlorobenzene: >99 Mix AU₃O₈ H₂O₂ Hexanes: >99 10 mls 0.11 g 1.5 mls Benzene: >99 Chlorobenzene:>99

The uncatalysed reaction showed a large increase in conversion comparedwith the uncatalysed reaction at room temperature, with conversions of85%, 84% and 92% for hexanes, benzene and chlorobenzene respectively.The catalytic activity of the U₃O₈ on the other hand seemed to be veryslightly reduced by increasing the temperature to 50° C. This may beexplained by an increase in the rate of thermal H₂O₂ decomposition bythe U₃O₈ relative to the rate of conversion of organic compound.

EXAMPLE 3

Mixture of Benzene, Chlorobenzene and Hexanes Using Air as Oxidant

A 10 ml batch of mix A was treated according to the method outlinedabove at ambient temperature except that air was used as the oxidantinstead of aqueous hydrogen peroxide. The air was held in a reservoir toprevent evaporation of the organic compounds. The conversions when usingunsupported U₃O₈ were >99% for hexanes, benzene and chlorobenzene, seeTable 3 below. Thus the conversions obtained by using air as oxidant arevery similar to those obtained by using hydrogen peroxide.

TABLE 3 Results for the oxidative destruction of hexanes benzene andchlorobenzene in water using air as oxidant. Substrate Catalyst Oxidant% Conversion Mix A U₃O₈ Air Hexanes: >99 10 mls 0.1 g Benzene: >99Chlorobenzene: >99

EXAMPLE 4

Benzene

A mixture was prepared of 2 mls benzene, 200 mls water and 5 mtst-butanol. 0.1 g of the catalyst and 2.5 mls H₂O₂ (30% w/w in water)were added to 20 mls of the mixture. The reaction mixture was stirredovernight (16 hrs) at room temperature (unless otherwise stated). Theresults for the catalysts used are specified in Table 4.

TABLE 4 % Conversion of Benzene over Uranium Oxide Catalysts. CatalystUsed % Conversion No catalyst, RT 0 U₃O_(8, 50° C.) >99U₃O_(8, 50° C. RT) >99 U on Al₂O_(3, RT) >99 U on SiO₂, RT 99

All the catalysts showed good activity for the destruction of benzene.There was no significant variation between the catalysts in performance.No visual changes to any of the catalysts were observed.

EXAMPLE 5

Chlorobenzene

A mixture was prepared of 2 mls chlorobenzene, 200 mls water and 5 mlst-butanol. 0.1 g of the catalyst and 2.5 mls H₂O₂ (30% w/w in water)were added to 20 mls of the mixture. The reaction mixture was stirredovernight (16 hrs) at room temperature (unless otherwise stated). Theresults for the catalysts used are specified in Table 5.

TABLE 5 % Conversion of Chlorobenzene over Uranium Oxide Catalysts.Catalyst Used % Conversion No catalyst, RT 0 U₃O_(8, 50° C.) >99U₃O_(8, 50° C. RT) >99 U on Al₂O_(3, RT) >99 U on SiO₂, RT >99

All the catalysts showed good activity for the destruction ofchlorobenzene. There was no significant variation between the catalystsin performance. No visual changes to any of the catalysts were observed.

The higher % conversion values for the aromatic compounds compared toother compounds such as TBP described below is probably due to the factthat aromatic compounds offer easier pathways to decomposition.

In addition, a silica supported uranium/vanadium oxide catalyst preparedas described above was tested.

The destruction of chlorobenzene using the silica supporteduranium/vanadium oxide catalyst is shown in Table 6.

TABLE 6 % Conversion of chlorobenzene against time using a silicasupported uranium/ vanadium oxide catalyst. Time (Mins.) % Conversion 00 40 57 80 79 160 82 260* 81 360 90 Extra H₂O₂ added

The uranium/vanadium oxide catalyst on silica shows very good activityfor the destruction of chlorobenzene. Conversion is close to 80% after80 mins. Conversion then essentially halts but can be seen to increaseagain once further peroxide is added. The reaction was left runningovernight and conversion increased to 97%.

EXAMPLE 6

Mixed Hexanes

A mixture was prepared of 2 mls mixed hexanes, 200 mls water and 5 mlst-butanol. The hexanes used were obtained from the hexane fraction ofpetroleum. 0.1 g of the catalyst and 2.5 mls H₂O₂ (30% w/w in water)were added to 20 mls of the mixture. The reaction mixture was stirredovernight (16 hrs) at room temperature (unless otherwise stated) Theresults for the catalysts used are specified in Table 7.

TABLE 7 % Conversion of Hexanes over Uranium Oxide Catalysts. CatalystUsed % Conversion No catalyst, RT 0 U₃O₈, 50° C. 65 U₃O₈, RT. >99 U onAl₂O_(3, RT.) 95 U on SiO_(2, RT.) 97

All of the catalyst showed good activity for the destruction of thehexanes. There was no loss of performance when the reaction is run atroom temperature compared with 50° C., in fact, the catalysts were moreactive at room temperature. The lower activity at 50° C. is probably dueto an increase in the rate of the hydrogen peroxide decomposition beforethe higher conversion could be achieved. No visual changes to any of thecatalysts were observed

EXAMPLE 7

Oxygen as Oxidant

Reactions were carried out under the same conditions as in Examples 4 to6 but no hydrogen peroxide was added. Instead, the reactions werecarried out in a three necked flask, with one neck closed, a reservoirconnected to one neck and the other neck used to fill the flask andreservoir with oxygen. The latter neck was then sealed, while keepingthe reservoir filled with oxygen. The mix was left stirring overnight atroom temperature. Results are shown in Table 8.

TABLE 8 Results for the destruction of organic compounds with oxygen asoxidant. Catalyst Organic % Conversion U₃O₈ Chlorobenzene >99 U₃O₈Hexanes 97 U₃O₈ Benzene 99 U/Cu/Fe on SiO₂ Chlorobenzene 88 None Hexanes25

Conversion was shown to be very high as with hydrogen peroxide. Table 8also shows the result for a silica supported uranium/copper/ironcatalyst prepared as described above. This showed good conversionalthough it was slightly lower than the simple uranium oxides.

EXAMPLE 8

Tri-butyl Phosphate (TBP)

2 mls of TBP were dissolved in 200 mls of water. To 20 mls of thissolution was added 0.1 g of catalyst and 5 mls of H₂O₂ (30% w/w inwater). The reaction mixture was then stirred overnight for 16 hrs atroom temperature (unless otherwise stated). The results are shown inTable 9.

TABLE 9 Results for the destruction of TBP. Catalyst Used % ConversionNo catalyst, RT  0 U₃O₈, RT 81 U on Al₂O_(3, RT) 80 U on SiO_(2, RT.) 72

The oxidation of TBP under the above conditions was investigated as afunction of time. The analysis was carried using a Perkin ElmerTurbomass GC-MS. To avoid saturation of the column and detector bywater, analysis was preceded by extraction of the organic from theaqueous phase into chloroform ( a sample of standard volume was taken atthe stated time and acidified with 2-3 drops of nitric acid, a standardvolume of chloroform was then added and the sample was then mixed usinga vortex mixer and then centrifuged to fully separate the two phase).Results at room temperature are shown in Table 10.

TABLE 10 Results for TBP destruction for time on line study. Time(Mins.) % Conversion 0  0 30  0 90 22 210 23 330 31 4320 78

The results indicate that whilst TBP destruction using a U₃O₈ catalystis efficient, the reaction could still be improved. Other catalysts werethen tested.

A mixed metal oxide catalyst was prepared comprising a silica supporteduranium/vanadium oxide catalyst. Vanadium was chosen as a suitablecomponent because vanadium oxide is known to bring about the rapiddecomposition of hydrogen peroxide. Silica was chosen as a support assilica supported catalysts are relatively easy to prepare. Theuranium/vanadium oxide on silica catalyst was prepared as describedabove. The results for the destruction of TBP at room temperature usingthe uranium/vanadium oxide on silica catalyst are shown in Table 11.

TABLE 11 Results for the destruction of TBP using thc uranium/vanadiumoxide on silica catalyst Time (Mins.) % Conversion 0  0 30 26 60 48 15050 210* 52 360 67 *Extra H₂O₂ added

The uranium/vanadium oxide on silica catalyst showed a dramatic increasein the rate of TBP destruction as against the U₃O₈ catalysts. Increasedeffervescence of the reaction mixture upon addition of the catalystindicated accelerated peroxide decomposition. The fall off in activityof the catalyst after 90 minutes is almost certainly due to theexhaustion of the hydrogen peroxide supply as the conversion rate risesonce more the addition of extra hydrogen peroxide. As mentioned above,the silica support may undergo some hydrolysis and a better support maybe alumina, titania or ceria for example.

Experiments with unsupported and alumina supported vanadium oxide, V₂O₅,alone (i.e. without any uranium) showed little or no conversion of TBP.This demonstrates the essential role of uranium in the presentinvention.

It can also be seen from the above that the efficiency of the uraniumcontaining catalyst can be increased by careful incorporation of otherelements such as vanadium. At least in the case of TBP oxidation, auranium/vanadium mixed oxide is very efficient.

To summarise the above TBP experiments, it appears that the unsupportedU₃O₈ and uranium oxide supported on alumina catalysts showed higherconversions than the corresponding silica supported catalyst. The silicasupported uranium/vanadium oxide catalyst showed a comparable conversion(67%) in less than half the time.

EXAMPLE 9

Treatment of a Simulated Effluent

Uranium containing catalysts were also investigated for the destructionof TBP present in a simulated effluent stream. Industrial effluentscontain many other contaminant species, metallic, organic and otherwise,and these other species may effect the activity of the catalysts.

The tested simulated effluent mix comprised low ppm levels of the metalsiron, nickel, copper, zinc, lead and uranium, low levels of thecomplexants citric acid and EDTA, TBP at 50 μl/l and co-present sodiumnitrate and sodium nitrite. The pH was converted to about 12 usingcaustic soda.

0.1 g of catalyst and 0.5 mls of 30% hydrogen peroxide was added to 20mls of the effluent mix. The reaction was stirred for 16 hours at roomtemperature. The results are shown in table 12. In addition, anexperiment with the uranium/vanadium oxide on silica catalyst wascarried as described but using a larger amount, 3 mls ,of 30% hydrogenperoxide.

TABLE 12 Destruction of TBP present in simulated effluent Catalyst %Conversion None 0 U₃O₈ 1 U on Alumina 1 U/V on silica 41 U/V on silica(High peroxide 78 conc.)

The simple uranium catalysts were not very efficient for the destructionof TBP in the simulated effluent mix. This might be due to the effect ofthe co-present metal ions. However, the uranium/vanadium oxide catalystwas very effective, especially at the higher peroxide concentrations. Auranium/vanadium oxide based catalyst thus appears to be particularlywell suited to the destruction of TBP in an industrial type effluent.

It should be noted that in other types of effluents, where theconditions and composition may be quite different from the simulated mixabove, the simple uranium oxide catalysts may be more effective for TBPdestruction than was found here.

The simple uranium oxide catalysts may be more effective for destroyingother organic compounds than TBP. The simple uranium oxide catalysts maybe more effective for destroying aromatics for example.

A uranium/ vanadium oxide catalyst is particularly effective fordestroying TBP.

From the foregoing results it is clear that for any particular organiccompound which it is desired to destroy, a particular uranium containingcatalyst may be specifically designed to efficiently catalyse thedestruction of that compound. For example, in the case of destruction ofbenzene, chlorobenzene or hexanes, a simple uranium oxide may be veryactive. In the case of TBP destruction, a uranium/vanadium oxidecatalyst is very active. Other potentially useful mixed metal oxidecatalysts include mixed metal oxides of uranium with e.g. iron orcopper.

The uranium containing catalyst may also be designed according to theconditions present in the effluent, e.g. according to the pH, the natureof the co-present metal ions and other components etc.

The embodiments discussed and described hereinabove do not limit theinvention in any way. Any catalyst containing uranium may be effectivefor catalysing the oxidative destruction of organic compounds in aqueoussolution.

In summary, the present invention provides a highly efficient method ofdestroying organic pollutants in aqueous effluents which can be readilycarried out at ambient temperature if desired, is capable of utilising asimple oxidant such as hydrogen peroxide or air and does not suffersignificantly from catalyst leaching.

What is claimed is:
 1. A method of destructively oxidising an organiccompound present in an aqueous solution, the method comprising oxidisingthe organic compound at a temperature below 100° C. in the presence of acatalyst which contains uranium.
 2. A method as in claim 1 and whereinthe temperature is below about 50° C.
 3. A method as in claim 1 or 2 andwherein the temperature is about ambient temperature.
 4. A method as inclaim 1 wherein the catalyst comprises one or more oxides of uranium. 5.A method as in claim 4 and wherein the catalyst has a stoichiometry inthe range from UO₂ to UO₃ inclusive.
 6. A method as in claim 5 andwherein the catalyst comprises U₃O₈.
 7. A method as in claim 1 whereinthe catalyst also comprises one or more other metals.
 8. A method as inclaim 7 and wherein the one or more other metals are selected from thegroup consisting of vanadium, copper, iron and platinum.
 9. A method asin claim 1 wherein the catalyst also comprises one or more other metaloxides.
 10. A method as in claim 9 and wherein the one or more othermetal oxides are selected from the group consisting of oxides ofvanadium, copper, iron and platinum.
 11. A method as in claim 1 whereinthe catalyst comprises a mixed metal oxide of uranium and at least oneother metal.
 12. A method as in claim 11 and wherein the mixed metaloxide comprises a uranium and vanadium mixed metal oxide.
 13. A methodas in claim 1 wherein the catalyst is supported on a support.
 14. Amethod as in claim 13 and wherein the support is selected from the groupconsisting of SiO₂, Al₂O₃, zeolites, activated carbon, TiO₂, ZrO₂, andCeO₂.
 15. A method as in claim 1 wherein the organic compound isoxidized by hydrogen peroxide.
 16. A method as in claim 1 wherein theorganic compound is oxidized by air or oxygen.
 17. A method as in claim1 wherein the organic compound comprises a compound selected from thegroup consisting of alkanes, alkenes, alkynes, aromatics, alcohols,aldehydes, ketones, carboxylic acids, esters, ethers, amines,detergents, organic phosphates and derivatives thereof.
 18. A method asin claim 17 and wherein the organic compound comprises an alkylphosphate.
 19. A method as in claim 18 and wherein the alkyl phosphateis tri-butyl phosphate.